A credit can be fully entered: Simply pick the most appropriate option ("other instruments" if nothing else) and enter the actual credit as text.

When a new instrument is added, we can easily search existing credits for the text to fix them.

When fixing them, it would be possible to comment on the original edit to let the user know that it's been added.

Even if it's too specific, you can enter the text.

Even if it's too vague, you can enter the text.

If instruments are added as entities (MBS-3674) or even if we only add instrument aliases (MBS-1028), this would mean the instrument requests page in its current form is no longer needed. Instead, we could simply generate a list of instrument credits which are not aliases of the instrument. That would be a list of things which are either valid aliases and should be added as aliases, or are actually separate instruments which should be added as instruments. The only time someone would need to request an instrument would be to split something which is currently entered as an alias of an existing instrument.